The renin-angiotensin system has diverse roles in the regulation of body fluid and electrolyte balance and blood pressure control. These actions are exerted in a variety of target organs, including the cardiovascular system, adrenal glands, kidney and central and peripheral nervous systems, by both the circulating hormone and hormone locally produced in tissues. Most of these actions are exerted by the octapeptide, angiotensin II, although the C-terminal heptapeptide angiotensin III has some activity. The hexapeptide NH2-Val Tyr Ile His Pro Phe-COOH (SEQ ID No:10), corresponding to the 3–8 fragment of angiotensin II (ie. amino acids 3–8), is also called angiotensin IV (Ang IV), and has until recently been believed to be an inactive degradation product devoid of biological activity.
However, Harding and co-workers have confirmed an earlier report (Braszko et al, 1988) that Ang IV has central nervous system activity, and can modify learning and behaviour (Wright et al, 1995). In addition, Ang IV has vasoactive effects, and can dilate cerebral arteries (Haberl et al, 1991) and increase renal blood flow (Swanson et al, 1992). This, coupled with the discovery of highly specific, high affinity sites for Ang IV binding in bovine adrenal and other tissues, has reawakened interest in the hexapeptide, and the subject has been comprehensively reviewed (Wright et al 1995).
Ang IV has been associated with the central nervous system effects of increasing stereotypy behaviour (Braszko et al, 1988) and facilitating memory retrieval in passive avoidance studies (Braszko et al, 1988; Wright et al, 1995). Ang IV also dilates cerebral arterioles (Haberl et al, 1991), and increases renal blood flow (Swanson et al, 1992).
Receptor autoradiographic studies have revealed a widely abundant but selective and characteristic distribution of binding sites for [125I]Ang IV (known as the AT4 receptor) in the guinea pig, sheep and monkey central nervous systems, in regions associated with cholinergic neurons and in somatic motor and sensory associated areas (Miller-Wing et al, 1993; Moeller et al, 1995, Moeller et al, 1996). In addition, Ang IV binding sites are abundant in supraspinal components of the autonomic nervous system, and in the spinal cord are found in sympathetic preganglionic neurons, in the dorsal root ganglia, and in Lamina II of the dorsal horn, and in the motor neurons of the ventral horn (Moeller et al, 1995).
The distribution of the Ang IV binding site differs from the localization of the Ang II AT1 or AT2 receptors. In addition, the pharmacology of each receptor is distinct in that the Ang IV site exhibits a low to very low affinity for [Sar1Ile8]Ang II, the non-subtype selective Ang II antagonist, and losartan (du Pont-Merck) and PD 123319 (Parke-Davis), the specific AT1 and AT2 receptor antagonists respectively (Miller-Wing et al, 1993; Swanson et al, 1992; Hanesworth et al, 1993). Conversely, Ang II receptors show a low affinity for the Ang IV binding site (Bennett and Snyder, 1976).
The wide distribution of the Ang IV binding site in motor, sensory and cholinergic regions suggests important roles for this peptide in the central nervous system. However, a physiological action of the peptide in neurons has yet to be clearly defined.
Numerous neurotransmitters and neuropeptides have been associated with the regulation of neuronal development. Acetylcholine inhibits neurite outgrowth from embryonic chicken ciliary ganglion cells and sympathetic neurons (Pugh and Berg, 1994; Small et al, 1995), and rat hippocampal neurons (Muttson, 1988). Conversely, vasoactive intestinal peptide stimulates superior cervical ganglion branching (Pincus et al, 1990) and somatostatin increases neuronal sprouting from Helisoma buccal ganglion neurons (Bulloch, 1987).
We have now surprisingly found that the peptide LVV-haemorphin-7, derived from β-globin, acts as an agonist at the AT4 receptor, and is the endogenous ligand for the AT4 receptors in the brain. We have characterised its pharmacological activity. This enables us to design novel agonists and antagonists of Ang IV action.